Post-partum, ovarian function in dairy cows as revealed by concentrations of oestradiol-17 β and progesterone in defatted milk

Concentrations of oestradiol-17 β and progesterone in defatted milk have been used to study the post-partum restoration of ovarian function in 52 autumn- calved dairy cattle. In 32 cows the first preovulatory peaks in oestradiol-17 β concentration (indicative of imminent ovulation) were ≤ 15 days and in 49 cows ≤ 49 days post partum. Delay to first ovulation was mainly due, not to failure of ovarian secretion of oestradiol-17 β at preovulatory level, but to failure of oestradiol-17 β at this level to exert its normal preovulatory function. Longer intervals to first ovulation were associated with longer intervals from calving to the clearance of placental oestradiol-17 β, high peak milk yields and high body weight losses, suggesting that in high-yielding dairy cows these factors (themselves inter-related) may be associated with others, which are the immediate cause of the inhibition of the normal, preovulatory function of oestradiol-17 β. Forty-eight per cent of cows had short, first ovulation cycles. Ovarian function, comprising oestradiol-17 β secretion at preovulatory level, normal preovulatory function of oestradiol-17 β and normal corpus luteum progesterone secretion were almost fully restored in this herd by the 49th day post partum.